The present disclosure relates to a differential transformer magnetic permeability sensor.
In an image forming apparatus using toner as a developer, a magnetic permeability sensor is used for detecting remaining toner amount or density. There are various types of the magnetic permeability sensor, and one of them is a differential transformer magnetic permeability sensor, which has a structure in which a drive coil, a detection coil, and a reference coil are arranged on the same core.
Using planar coils, the differential transformer magnetic permeability sensor can be downsized. For instance, as an example of the differential transformer magnetic permeability sensor using planar coils, there is conventionally proposed a structure including a first coil (drive coil) disposed in a first layer, a second coil (reference coil) disposed in a second layer, a third coil (detection coil) disposed in a third layer, a fourth coil (drive coil) disposed in a fourth layer, and insulating substrates disposed between layers.
However, the differential transformer magnetic permeability sensor using planar coils as described above may cause a position shift in forming the drive coil, the detection coil, and the reference coil due to an error in the manufacturing process or the like. As a result, in a state where there is no magnetic substance to be detected, an amount of magnetic flux passing through the detection coil and an amount of magnetic flux passing through the reference coil cannot be equalized, and therefore accuracy of measurement is deteriorated. In particular, when a shift amount is different in each sensor, in a state where there is no magnetic substance to be detected, a difference between the amount of magnetic flux passing through the detection coil and the amount of magnetic flux passing through the reference coil is different in each sensor. As a result, a variation of output level occurs in each sensor, and hence accuracy of measurement is deteriorated.
Accordingly, for example, there is already proposed a differential transformer magnetic (magnetic permeability) sensor in which a first differential coil (reference coil) constituted of a flat winding wire and a first drive coil constituted of a flat winding wire parallel to the wire constituting the first differential coil are arranged on a first surface of an insulating single layer substrate, while a second differential coil (detection coil) constituted of a flat winding wire and a second drive coil constituted of a flat winding wire parallel to the wire constituting the second differential coil are arranged on a second surface of the substrate.
In this parallel flat winding differential transformer magnetic sensor, a plurality of first branch lines are further disposed so as to branch from the wire forming the outermost periphery of the first differential coil and are arranged so that an amount of magnetic flux passing through each of the first branch lines is different when the first drive coil is driven. In the same manner, a plurality of second branch lines are disposed so as to branch from the wire forming the outermost periphery of the second differential coil and are arranged so that an amount of magnetic flux passing through each of the first branch lines is different when the second drive coil is driven. Further, one of the plurality of first branch lines and one of the plurality of second branch lines are selectively used so as to take a balance between an electromotive force generated in the first differential coil and an electromotive force generated in the second differential coil (to perform zero adjustment).
However, in the parallel flat winding differential transformer magnetic permeability sensor described above, the wire constituting the reference coil and the wire constituting the first drive coil are arranged in parallel to each other. In the same manner, the wire constituting the detection coil and the wire constituting the second drive coil are arranged in parallel to each other. In other words, capacitance between the reference coil and the first drive coil as well as capacitance between the detection coil and the second drive coil is large, as if capacitors are connected respectively between the reference coil and the first drive coil and between the detection coil and the second drive coil, with electrodes that are the wires constituting the coils.
As a result, in the parallel flat winding differential transformer magnetic permeability sensor described above, a voltage variation of the drive coil can easily cause voltage variations of the reference coil and the detection coil via the capacitors between the wires. In this way, the voltages generated in the reference coil and the detection coil have phases different from those of the voltages generated in the reference coil and the detection coil when current flows in the drive coil, and hence the voltages cannot be reduced even if any branch line described above is selected.